Phase detector circuit



March 14, 1967 D. R. BROSIOUS PHASE DETECTOR CIRCUIT 7 Sheets-Sheet 1 Original Filed Aug. 6, 1963 ELECTRODE mvsw'rox Dan/ R. Bros/'00s March 14, 1967 D. R. BROSIOUS PHASE DETECTOR CIRCUIT 7 Sheets-SheetP,

Original Filed Aug. 6, 1963 INVENTOR. Daniel R Bros/bus March 14, 1967 D. R. BROSIOUS PHASE DETECTOR CIRCUIT 7 Sheets-Sheet 5 Original Filed Aug. 6, 1963 INVENTOR. Danie/R. Bros/bus March 14, 1967 D. R. BROSIOUS 3,309,539

PHASE DETECTOR CIRCUIT Original Filed Aug. 6, 1963 7 SheetsSheet 4 Hg. 7 1am V W INVENTOR. r2

Dame/ R. Bros/00s D. R. BROSIOUS 3,309,539 PHASE DETECTOR CIRCUIT I 1963 7 Sheets-Sheet 6 March 14, 1967 Original Fiied Aug. 6

INVENTOR Dan/'e/ R Bros/'00s United States Patent 8 Claims. (Cl. 30788.5)

This application is a division of copending application Ser. No. 300,211 filed August 6, 1963, for Eddy Current Seam Tracking Device.

This invention relates to apparatus for electromagnetically locating a seam or discontinuity in conductive materials. More particularly, it relates to apparatus for inducing eddy currents of a predetermined distribution in conductive materials and detecting any departure from said predetermined distribution as the apparatus is moved relative to the materials. In addition, this invention relates to a novel phase detector circuit which produces a DC. signal the magnitude and polarity of which are dependent upon the magnitude and direction of phase shift of an A.C. signal.

The invention will be described in detail in connection with automatic positioning devices. For example, in the automatic welding of a seam it is desirable to maintain the welding electrodes centered with respect to the seam as the welding agency is moved relative to the materials forming the seam. It is thus necessary to provide sensing means for continuously producing an electrical signal indicative of the position of the welding electrodes with respect to the seam and further means which utilizes said signal to automatically re-position either the welding agency or the materials forming the seam when said signal indicates that the electrodes are not centered Wit respect to the seam.

The sensing means of the invention produces an A.C. signal which varies in phase as the position of the electrodes varies with respect to the seam. In order to accurately sense small changes in position it is essential that circuit means, e.g. a phase detector circuit, be provided which produces a large DC. signal when said A.C. signal shifts in phase through a small angle, e.g. a few degrees. In addition, said circuit should produce a signal indicative of the magnitude and direction of phase shift. 7

It is an object of this invention to provide an apparatus for inducing eddy currents of a predetermined distribution in conductive materials and detecting any departure from said predetermined distribution as the apparatus is moved relative to the materials. It is a further object to provide such an apparatus which is relatively uninfluenced by electrical noise and other amplitude phenomena.

It is a still further object of the invention to provide an apparatus for continuously centering a member with respect to a seam formed by conductive materials.

' It is another object of the invention to provide an apparatus which produces an A.C. signal which varies in phase as the eddy current distribution varies in the materials under test.

A further object is to provide a phase detector circuit which produces a relatively large DC. signal when an A.C. test signal shifts in phase through a relatively small angle. A still further object is to provide such a circuit in which the magnitude and polarity of said DC signal are dependent upon the magnitude and direction of phase shift.

To achieve the foregoing objects, I provide an eddy current probe comprising a ferromagnetic E-shaped core having a base, an inner leg, and first and second outer legs. First coil means comprising an excitation coil is wound about said inner leg and second coil means comprising first and second pick-up coils are wound about 3,369,539 Patented Mar. 14, 1967 ice said first and second outer legs, respectively. The excitation coil is connected to a source of alternating current and the two pick-up coils are connected, in series-opposition, across a capacitor to form a parallel resonant circuit resonant at the frequency of said alternating current. A plurality of circuits, to be described hereinafter, connects the resonant circuit to a phase detector circuit which produces a DC signal the magnitude and polarity of Which are dependent upon the difference in phase between the voltage across the resonant circuit and a reference voltage of the same frequency.

The probe is positioned in inductive relation with conductive materials forming a seam. The probe is disposed with its long axis transverse to the seam and with its inner leg centered directly above the seam. The excitation coil is then energized so that eddy currents are induced in said materials. When the inner leg is centered directly above the seam, the eddy currents on each side of the seam are substantially equal in magnitude. The probe is constructed so that there is no net voltage induced in said pick-up coils by the magnetic flux produced by said eddy currents when the probe is so positioned. The probe is further constructed so that a net voltage of minimum magnitude is always across the pick-up coils by reason of the magnetic flux produced by the current in the excitation coil.

Because of the hysteresis in the materials, and also because of the inductance of the coils, the voltage induced in said pick-up coils by the excitation current and the voltage induced in said pick-up coils by said eddy currents are out of phase with each other. Thus, the net voltage across said pick-up coils, and consequently across the resonant circuit, is the vectorial sum of said voltages.

The net voltage across the resonant circuit is fed through a plurality of circuits to a phase detector circuit which compares the phase of said voltage with that of a reference voltage of the same frequency. The phase detector circuit is constructed so that there is no output therefrom when the net voltage across the resonant circuit is due to flux produced by the excitation current only. However, when the net voltage across the resonant circuit is due to the combined effects of the excitation current flux and the eddy current flux, said voltage difiers in phase from the voltage induced in said pick-up coils by the excitation current flux only, and the phase detector circuit produces a DC. signal the magnitude and polarity of which are indicative of the magnitude and direction of the phase difference. Such a condition exists when the inner leg of the probe is not centered directly above the seam, inasmuch as the eddy currents on each side of the seam are then not substantially equal in magnitude, and the voltages induced in said pick-up coils by the magnetic flux produced by said eddy currents are consequently unequal in magnitude.

The features of the invention which I believe to be novel are set forth with particularity in the appended claims. Both the structure and the method of operation of the invention, as well as further objects and advantages thereof, will be better understood by reference to the following description taken in connection with the drawings, in which FIGURE 1 is a side elevational view of an arc welding apparatus including a positioning control apparatus embodying certain of the principles of the present invention.

FIGURE 2 is a sectional view of said apparatus taken along the line 2-2 of FIGURE 1.

FIGURE 3 is a side elevational view, partly in section, of a supporting apparatus for an eddy current probe.

FIGURE 4 is an end elevational view of the eddy current probe disposed in inductive relation with conductive materials forming a seam.

FIGURE 5 is a vector diagram of the fluxes in the materials due to eddy currents induced therein.

FIGURE 6 is a diagram of the waveforms of several of the fluxes linking the eddy current probe.

FIGURE 7 is a vector diagram of the voltages across the probe.

FIGURE 8 is a block diagram of the circuits to which the probe is connected.

FIGURE 9 is a schematic diagram of the circuits of FIGURE 8.

FIGURE 10 is a diagram of the input and output voltages of the phase detector circuit when the eddy currents induced in the materials forming the seam are of the desired distribution.

FIGURE ll is a diagram of the input and output voltages of the phase detector circuit when the eddy currents induced in the materials are not of the desired distribution.

As is shown in FIGURES l and 2, the positioning control apparatus broadly comprises a supporting structure 10 which is disposed above a pipe 11 the longitudinal seam 12 of which has been welded along the inside thereof and has just begun to be welded along the outside thereof. The structure 10 comprises side plates 13 and 14, end plates 15 and 16, and cover plate 17. The cover plate 17 is welded to a bracket 18 which depends from a suitable support (not shown).

Journaled within the end plates 15 and 16 are rods 19 and 2t Motive power means, e.g. a reversible A.C. motor 21, is affixed to a motor mounting plate 22 which is riveted to the end plate 16. The shaft of the motor 21 is adapted to rotate a worm 23 which is journaled in bushings 24 and 25. The worm 23 thread-edly engages a nut 26 which is rigidly attached to the bottom of a carriage 27. The carriage .27 is provided with bushings 28 and 29 in which rod 19 is journaled and bushings 31 and 31 in which rod is journaled. Rotation of the shaft of the motor 21 thus drives the carriage 27 along the rods 19 and 2t Attached to the bottom of the carriage 27 is a U- bracket 32 to which the probe supporting apparatus 33 is aflixed. The apparatus 33, shown in detail in FIGURE 3, broadly comprises a circular head 34 having an annular collar 35. The head 34 is provided with an opening 36 through which air is supplied by a flexible hose 37. A bellows 38 is attached to the bottom of the annular collar by means of a sealing ring 39. The bottom of the bellows 38 encircles a supporting ring 46 having a circumferential groove 41. The bellows 38 is affixed to the ring 40 by mean of a ring clamp 42. A shield 43 having an annular flange 4 5 is circumferentially disposed about the bottom of the bellows 38 by means of the ring clamp 45. The outer edge of the flange 44 is adjacent the inner surface of the cylindrical casing 46 which is bolted to the collar 35. :The bellows 38 is thus protected from damage due to sparks, rough handling, etc.

A trunnion fork 47 is bolted to the bottom of the ring 40 and is provided with journals 48. A sensing head 49 is pivotally mounted in the fork 47 by means of pins 50 inserted in the journals 48.

The sensing head 49 broadly comprises a body section 51 in which the eddy current probe 52 is housed. The body section 51 has two cylindrical projections 53 and 54 about which wheels 55 and 56 are placed. The ends of said projections are threaded whereby nuts 57 and 58 can be secured thereto.

Returning to FIGURE 1, a plate 59 is shown depending from one side of the carriage 27. Another plate 60 extends horizontally from the plate 59 and provides mounting means for the electrodes 61. The electrodes 61 are automatically fed to the seam 12 by means of the electrode feed apparatus 62, which is rigidly mounted in the vicinity of the supporting structure 19.

Means, not shown, are provided for driving the pipe 11 longitudinally whereby the seam 12 of the pipe 11 progressively passes under the structure 10. Further means, also not shown, are provided for rotating the pipe 11 whereby the seam 12 is sulbstantialy in the vicinity of the electrodes 61. Inasmuch as the electrodes 61 may not be centered directly above the seam 12, it is necessary to sense the position of the seam 12 with respect to the electrodes 61 and re-position the carriage 27 if the eddy current probe 52 and its associated circuits indicate that re-positioning is desired. The electrical connectors between the probe 52 and its associated circuits are encased in a flexible tube 63 which is affixed to a connector block 64. The block 64 is mounted on the casing 46 and is provided with flexible conduits 65 and 66 through which the connectors pass to the probe 52.

Broadly, the above-described apparatuses operate in substantially the following manner. The trunnion fork 47 is retracted within the casing 46 by removing the air in the bellows 38 through the hose 37. The pipe 11 is then driven longitudinally and rotated until the beginning of the seam is substantialy under the electrodes 61. The fork 47 is then lowered, by supplying air through the hose 37, until the wheels 55 and 56 of the sensing head 49 rest on the surface of the pipe 11. The eddy current probe 52 is excited and produces an A.C. signal the phase of which is dependent upon the position of the probe 52 with respect to the seam 12. The signal from the probe 52 is utilized to energize the motor 21 to drive the carriage 27 along the rods 19 and 20 until the probe 52 is centered directly over the seam 12. By reason of the aforementioned mechanical linkage between the probe 52 and the electrodes 61, said electrodes are then also centered directly over the seam 12.

The welding current is then turned on and the pipe 11 is driven longitudinally whereby the pipe 11 progressively passes under the structure 10. Should the transverse position of the seam vary along the length of the pipe, thereby causing a lateral displacement of the seam 12 with respect to the electrodes 61, there is a corresponding lateral displacement of the seam 12 with respect to the probe 52. Said displacement alters the eddy current distribution in the pipe 11 adjacent the seam 12 thereof. That is, the eddy currents increase in magnitude on one side of the seam 12 and decrease in magnitude on the other side of the seam 12. The change in eddy current distribution results in a phase shift in the voltage across the pick-up coils, the magnitude of the phase shift being dependent upon the magnitude of the change in eddy current distribution, the hysteresis of the pipe material, and the inductance of the probe, as will be described in detail later in the specification. The phase shift in the voltage across the pickup coils is converted by a plurality of circuits, including a phase detector circuit, into a DC. signal which is fed to a motor control circuit. The motor control circuit, in response to said signal, energizes the motor 21 which drives the carriage 27 laterally with respect to the seam 12 until the probe 52, and thus the electrodes 61, is centered directly above the seam 12.

The eddy current probe 52 of the invention is shown in detail in FIGURE 4 as comprising a symmetrical E- shaped core 67 having a base 68, an inner leg 69, and first and second outer legs 70 and 71, respectively. It is essential to the invention, for reasons to be later explained, that the core 67 be constructed of a low Q ferromagnetic material, e.g. low grade silicon sheet steel.

The geometry of the core 67, which preferably is laminated to reduce the magnitude of eddy currents induced therein, is such that the reluctance between the inner leg 69 and the pipe 11 is substantially greater than the reluctance between either outer leg 70', or 71, and the pipe 11. This geometric relationship is preferably attained by providing the outer legs 70' and 71 with integral pole pieces 72 and 73, respectively, the cross-sectional areas of which are large in comparison with the cross-sectional area of the inner leg 69.

Wound about the inner leg 69 is an excitation coil 74.

Wound about the outer legs 70 and 71 are low Q pick-up coils 75 and 76, respectively. Coils 75 and 76 are connected in series-opposition, insofar as voltages induced therein by the excitation fluxes produced by alternating current in coil 74 are concerned. It is essential to the invention that the coils 75 and 76 be electromagnetically unequal whereby different voltages are induced in said coils by the excitation fluxes. The electromagnetic inequality can be achieved by providing different magnetic paths for each pick-up coil, by providing each pick-up coil with a different number of turns, or by providing a combination of different magnetic paths and a different number of turns. In the preferred embodiment of the invention, said inequality is obtained by adjusting the relative positions of the pick-up coils on the outer legs (ditfering the magnetic paths) and by winding the pick-up COilS so that one of said pick-up coils has from one to five, and preferably three, percent less turns than the other pick-up coil.

The probe 52 operates in substantially the following manner. Alternating current, the frequency of which is determined by the electrical characteristics of the materials forming the seam 12 and the desired depth of penetration of the eddy currents to be induced therein, is fed to the excitation coil 74. A frequency within the very low frequency (VLF) range, e.g. 5000 c.p.s., has been found to be optimum for as-rolled mild steel pipe inch' thick in which the inside weld has penetrated 80%.

Alternating current in the excitation coil 74 results in fluxes in the core 67 of the probe 52 and the pipe 11. For purposes of analysis, the pertinent fluxes are shown in FIGURE 4.

The main flux M, i.e. the flux resulting from the alternating current in the excitation coil 74, is represented in terms of several component fluxes. Two components, M1 and M2, leave the inner leg 69 of the core 67 and inductively couple the probe 52 to the pipe 11. When the probe 52 is properly positioned with respect to the seam 12, qSMl enters the pipe 11 on one side of the seam 12 while M2 enters the pipe 11 on the other side of the seam 12. M1 and M2 leave the pipe 11 and return to the core 67 through pole pieces 72 and 73, respectively. By reason of the symmetry of the core 67, M1 and M2 are substantially equal in magnitude and in phase within the excitation coil 74, and are substantially equal in magnitude and 180 out of phase within the electrically connected pick-up coils 75 and 76.

Another two components, M11 and M22, of the main flux M leave the inner leg 69 and pass directly to the pole pieces 72 and 73, respectively. Said components, which do not contribute to the inductive coupling between the probe 52 and the pipe 11, are substantially equal in magnitude and in phase within-the inner leg 69 and substantially equal in magnitude and 180 out of phase within the electrically connected pick-up coils 75 and 76. An additional component L2 is a leakage flux which leaves the inner leg 69 and returns directly thereto via air.

The foregoing can be expressed mathematically as:

Inasmuch as one of the primary functions of the probe 52 is to induce eddy currents in the pipe 11, fluxes M1 and M2 should be relatively large with respect to fluxes M11 and M22. The outer legs 70 and 71 are thus so disposed with respect to the inner leg 69 that, when the sensing head 49 is resting on the pipe 11, the reluctance between each of the pole pieces 72 and 73 and the leg 69 is substantially greater than the sum of the reluctances between the leg 69 and the pipe 11 and either pole piece 72 or 73 and the pipe 11.

The fluxes produced by the current in the pick-up coils 75 and 76 due to the voltages induced therein by components of the main M passing therethrough are 6 labeled in FIGURE 4 as pLl and L3, respectively. The net flux L within the pick-up coils due to said current is substantially equal to the vector sum of L1 and L2.

When the probe 52 is inductively coupled to the materials forming the seam 12, the alternating fluxes Ml and M2 induce in the materials eddy currents the magnitudes of which may be large if the electrical conductivity of the materials is high. The eddy currents are confined to areas adjacent the seam 12 by supplying the coil 74 with current of a frequency for which the resultant eddy currents are restricted substantially to the surface of the pipe 11. The eddy currents thus cannot cross the seam 12 via the inside longitudinal weld 77. For the purpose of analysis, the individual eddy currents induced on each side of the seam 12 have been integrated and are represented in dotted lines as 78 and 79, respectively.

The eddy currents 78 and 79, which are 180 out of phase with each other, produce fluxes R1 and R2, respectively. Said fluxes, which would be substantially 180 out of phase with fluxes M1 and M2, respectively, were the materials forming the seam 12 not characterized by hysteresis, are shown as vectors in FIG. 5. Flux R1 comprises a component E1 180 out of phase with M1 and a quadrature component H1 due to hysteresis. Similarly R2 comprises a component E2, 180 out of phase with M2 and a quadrature component H2 due to hysteresisj Flux R is the resultant eddy current flux across the seam 12 when R1 and R2 are unequal, a condition which exists when the inner leg 69 of the probe 52 is not centered directly above the seam 12. Mathematically expressed, R is the vector sum of Rl and R2.

It is to be noted that the paths of fluxes R1 and R2 do not include the inner leg 69 of the probe 52 and also that the path of each of said fluxes includes both of the outer legs 70 and 71. Flux R thus passes through the pick-up coils 75 and 76 in such a direction that the voltages induced in said coils by said flux add rather than subtract. Thus, while the pick-up coils 75 and 76 are connected in series-opposition insofar as the voltages induced therein by the main flux M are concerned, the pick-up coils 75 and 76 are connected in series-aiding insofar as the voltages induced therein by the resultant eddy current flux R are concerned.

FIGURE 6 diagrammatically shows the approximate Waveforms of the pertinent fluxes which pass through the pick-up coils 75 and 76. FIG. 6A shows the phase relationship between the main flux gaM and the net flux goL across the electrically connected pick-up coils 75 and 76. The net flux 01. is either in phase with M or, as shown, 180 out of phase with M. FIG. 6B shows the two components goER and (pEH of the resultant flux rpR produced by the integrated eddy currents 78 and 79. It is noted that component ER is 180 out of phase with the main flux (pM and that component c HR lags components ER by FIGURE 6C shows the waveform of the resultant eddy current flux (pR obtained by adding the components (pER and gal-IR. It can be seen that thephase of R differs from that of L by an angle X.

FIGURE 7 is a vector diagram showing the pertinent voltages across the excitation coil 74 and the pick-up coils 75 and 76 of the probe 52. E is the excitation voltage applied across the main field coil 74. E and E are the total voltages induced in the coils 75 and 76, respectively, by the main flux M. The net voltage E across the electrically connected coils 75 and 76 due to flux goM is the vector sum of E and E E is in phase with E inasmuch as E is of greater magnitude than E As has previously been mentioned, when the seam 12 of the pipe 11 is not directly below the inner leg 69 of the probe 52, there is a resultant flux goR through the pick-up coils 75 and '76 due to the inequality of the eddy currents 78 and 79 induced in the pipe 11. The flux (pR passing through the coils 75 and 76 induces voltage E in coil 75 and voltage E a in coil 76. Voltages E and E are in phase with each other and are approximately equal in magnitude, provided the degree of electromagnetic inequality between coils 75 and 70 is small. Said voltages are out of phase with the excitation voltage E by an angle X. The total voltage E across the electrically connected pick-up coils 75 and 76 is equal to the vector sum of E,,;,, E and E and'is out of phase with both E and the excitation voltage E by an angle B. The magnitude of E is relatively unimportant, provided it never decreases below the minimum level detectable by the associated electrical circuits. It is the angle B which is utilized by said circuits to indicate a lateral displacement of the seam 12 with respect to the probe 52.

The total voltage E across the pick-up coils 75 and 76 due to the alternating fluxes therein is very small. In order to obtain a useful signal, a capacitor 80, shown in FIGURE 9, is connected in parallel with the seriesconnected coils 75 and 76 to provide a resonant circuit 81 tuned to the frequency of the excitation voltage E It is essential to the invention that the resonant circuit 81 be of high quality (high Q), i.e. the circuit is characteriZed by a narrow band-width and the ratio of the reactance in the circuit to the resistance in the circuit is high.

While a high Q resonant circuit is generally constructed of high Q components (coils, cores, and capacitors), it is essential to the instant device that the pickup coils 75 and 76 and the E-shaped core 67 be of low Q. It is thus necessary for capacitor 80 to be of high Q in order for the circuit 81 to be of high Q. Summarizing, the high Q resonant circuit 81 comprises a high Q capacitor connected in parallel with a pair of low Q coils connected in series and wound about the outer legs of a low Q E-shaped core.

The Q of the core 67 must be low to insure that the Qs of the pick-up coils 75 and 76 are low. In addition, a low Q core tends to prevent substantial changes in the inductances of the coils 75 and 76 due to changes in the magnitudes of the fluxes passing through legs 70- and 71 and thus prevents de-tuning of the resonant circuit 81.

The pick-up coils 75 and 76 are of low Q for reasons which will be clear from a reconsideration of FIG. 6. A high Q coil which is being driven at a predetermined frequency tends to pick up signals only if said signals are of the same frequency as that of the driving signal and are either in phase or 180 out of phase therewith. Inasmuch as the pick-up coils 75 and 76 are being driven by voltages induced therein by flux components in phase with the main field flux M, it can be seen from FIG. 6B that only voltages induced in coils 75 and 76 by the component qoER of the resultant eddy current flux R will be picked up, as ER is 180 out of phase with the main flux M. The component flux (pHR, which is 90 out of phase with both the component QDER and the main flux M will not be picked up by coils 75 and 76. Inasmuch as the phase shift B which is utilized by the associated electrical circuits to control the motor 21 is due primarily to the component goHR, the successful operation of the invention is predicated upon the ability of the pick-up coils 75 and 7 6 to accept voltages in quadrature with the driving voltage. The acceptance of said quadrature voltages is assured by providing the probe 52 with low Q pick-up coils. In addition, the low Q prevents any change in the inductances of the pick-up coils 75 and 76 due to changes in the magnitudes of the fluxes passing through legs 70 and 71 and thus prevents de-tuning of the resonant circuit 81. The Q of the coils 75 and 76 should be no higher than 4, and should preferably be .01 or less.

The circuits associated with the probe 52 are shown in block diagram form in FIGURE 8. Broadly, the circuits comprise an oscillator 82 which produces a sinusoidal excitation voltage. The oscillator output voltage is amplified by a conventional voltage amplifier 83 and a power amplifier 84 and a portion of the amplified signal is fed to the excitation coil 74 of the probe 52. The remainder of the amplified signal is fed to a wave shaping circuit, e.g. a Schmitt trigger 85, which functions to convert the sinusoidal output from the power amplifier 84 into a square wave of identical frequency. The square wave is fed directly to a phase detector circuit 86 to provide a reference signal with which the phase of the signal across the resonant circuit 81 is compared.

The signal across the resonant circuit 81 is fed through high and low pass filters 87 and 88, respectively, which remove substantially all frequency components other than the 5000 c.p.s. component and shift the fundamental in phase by approximately The filtered signal is then amplified by a voltage amplifier 89 and fed to an emitter follower 90 which couples the signal to a square wave shaper 91. A portion of the output signal from the square wave shaper 91 is fed directly to the phase detector 86, while a second portion is fed through a phase inverter circuit 92 and hence to the phase detector 86.

The output from the phase detector 86 is zero when the probe 52 is properly positioned with respect to the seam 12. When the probe 52 is not properly positioned, the phase detector 85 develops an output signal the magnitude and polarity of which are dependent upon the magnitude and direction of the phase shift of the signal across the resonant circuit 81. In the preferred embodiment of the invention, the output signal from the phase detector 86 is fed to a difference amplifier 93 having two sensitive relays as loads. A positive signal from the phase detector 86 results in the actuation of one of the relays while a negative signal results in the actuation of the other. The relays control the energization of the reversible AC motor 21.

A detailed description of the circuits shown as blocks in FIG. 8 will be made with reference to the schematic diagram of FIGURE 9. The oscillator circuit 82 comprises a PNP transistor 94 having an emitter 95, a base 96, and a collector 97. The collector 97 is connectedto a tank circuit comprising the coil 98 and the capacitors 99 and 100. The junction 101 of the capacitor and the coil 98 is connected to a source 102 of negative D.C. potential, e.g. -30 volts. Bias resistors 103, 104 and 105 are provided to establish the D.C. operating point of the transistor 94. A resistor 106 and a capacitor 107 couple a portion of the oscillator voltage to the voltage amplifier 83.

' The voltage amplifier 83, which operates as a degenerative class A amplifier, comprises a PNP transistor 108 having an emitter 109, a base 110, and a collector 111. Resistors 112, 113, and 114 are provided to establish a DC. operating point for the transistor 108. Resistor 114 is unbypassed so that the input impedance to the amplifier 03 is high and so the operating characteristics of the amplifier 33 are linear. Resistor 115 is connected from the source 102 to the collector 111 and constitutes the amplifier load.

The power amplifier 34 comprises a PNP transistor 116 having an emitter 117, a base 118, and a collector 119. The output signal from the voltage amplifier '83 is coupled to the base 118 of the transistor 116 through a resistor 120 and a capacitor 121. Resistors 122, 123, and 124 are provided to establish a DC. operating point for the transistor 116. Resistor 124 is bypassed by a capacitor 125 to prevent variations in emitter bias due to variations in load current. The collector 119 is con nected to one side of the excitation coil 74 of the probe 52. The other side of the coil 74 is connected to a source 126 of negative potential, e.g. -50 volts. Due to the inductance of the probe 52, a capacitor 127 is connected across the coil 74 to prevent damage to the transisto-r 116 and to insure that the voltage across the coil 74 is substantially sinusoidal. The parallel combination of the coil 74 and the capacitor 127 provides the load for the transistor 116.

A portion of the signal across the excitation coil 74 is coupled to the Schmitt trigger 85 by a capacitor 128.

9 The Schmitt trigger comprises a PNP transistor 129 having an emitter 130, a base 131, and a collector 132 and a PNP transistor 133 having an emitter 134, a base 135, and a collector 136. A resistor 137 and a potentiometer 138 form a voltage divider network. The tap 139 of the potentiometer 138 is connected'to the base 131 of the transistor 129 and provides means for adjusting the symmetry of the output signal from the Schmitt trigger 85. The tap 139 is connected through a diode 140 to ground. The diode 140, in combination with the resistor 137 and the potentiometer 138, provides means for determining the voltage level which initiates conduction in the transistor 129. The emitters 130 and 134 of the transistors 129 and 133, respectively, are connected to a common bias resistor 141. A resistor 142 provides the load for the transistor 133 and a resistor 143 provides the load for the transistor 129. The resistor 143 is also one branch of the voltage divider network comprising the resistors 143, 144, and 145.

The components of the Schmitt trigger 85 are chosen so that, in the absence of a sufficiently negative signal at the base 131 of the transistor 129, transistor 129 is cut off and transistor 133 is in saturated conduction. A sufficiently large negative signal at the base 131 of the transistor 129, however, reverse biases the diode 140 and forward biases the transistor 129. The increasing load current through the resistor 143 results in a decrease of the forward bias on the base 135 of the transistor 133. Regenerative feedback between transistors 129 and 133 continues until transistor 129 is in saturated conduction and transistor 133 is cut off.

The Schmitt trigger 85 converts the input signal, which is substantially sinusoidal, to a square wave of the same phase and frequency as that of the sinusoid. The square wave output is coupled to the phase detector circuit 86 by a capacitor 146 and a leakage resistor 147. The square wave provides a reference signal with which the phase of the voltage across the resonant circuit 81, which is also converted to a square wave, is compared. By using square wave inputs to the phase detector 86, the detector output is unaffected by variations in the amplitudes of the input signals.

The signal across the resonant circuit 81 is fed through an active unsymmetrical band pass filter comprising a high pass filter 87 and a low pass filter 88. The high pass filter 87 comprises two identical sections. The first section comprises a PNP transistor 148 having an emitter 149, a base 150, and a collector 151. Resistors 152 and 153 are provided to establish the D.C. operating point for transistor 148 and resistor 154 is provided as the load. The remaining components of the filter are capacitors 155 and 156 and feedback resistor 157.

Transistor 148 functions as an emitter follower and is characterized by a high input impedance and a low output impedance. The input impedance is determined primarily by the combination of resistors 152 and 153. The combination of resistors 152 and 153 and capacitor 156 comprises a first R-C network. The combination of resistor 157 and capacitor 155 comprises a second R-C network. The values of the components of each R-C network are selected so that the time constant of each network is equal and the capacitive reactance is equal to the resistance at the frequency of the excitation voltage, viz. 5000 cycles per second.

The second section of the high pass filter 87 is identical to the first and comprises a PNP transistor 158 having an emitter 159, -a base 160, and a collect-or 161. Resistors 162 and 163 are provided to establish the D.C. operating point of transistor 158 and a resistor 164 is provided as the load. Capacitors 165 and 166 and feedback resistor 167 are components of the R-C networks.

Each high pass section provides an attenuation of 12 db per octave below 5000 cycles per second. Inasmuch as the welding current is 60 cycles per second, voltages induced in the pick-up coils 75 and 76 by the welding current are attenuated by more than db by the two high pass sections.

The low pass filter 88 is similar to one section of the high pass filter 87 and comprises a PNP transistor 168 having an emitter 169, a base 170, and a collector 171. Resistors 172 and 173 are provided to establish a D.C. operating point and a resistor 174 is provided as the load. Resistors 175 and 176 and capacitors 177, 178, and 179 are components of the R-C networks. The low pass filter 88 provides an attenuation of 12 db per octave above 5000 cycles per second and filters out the second harmonic of the excitation voltage as Well as spurious higher order frequencies generated by the welding current.

The band pass filter, besides filtering out undesired frequency components, serves to shift the phase of the 5000 c.p.s. signal by 90.

The output signal from the low pass filter 88 is coupled to a class A voltage amplifier 89 by a capacitor 180. The amplifier 89 comprises a PNP transistor 181 having an emitter 182, a base 183, and a collector 184. Resistors 185, 186 and 187 are provided to establish the D.C. operating point and a resistor 188 is the load. Capacitor 189 is an AC. bypassing capacitor for the emitter resistor 187.

The signal from the voltage amplifier 89 is coupled to an emitter follower 90 by a capacitor 190. The emitter follower 90 comprises a PNP transistor 191 having an emitter 192, a base 193, and a collector 194. Resistors 195 and 196 are provided to establish the D.C. operating point of transistor 191 and resistor 197 is the load. The emitter follower 90 functions to match the impedance between the output of the voltage amplifier 89 and the input of the square wave shaper 91.

The square wave shaper 91 comprises a PNP transistor 198 having an emitter 199, a base 200, and a collector 201. The signal from the emitter follower 90 is coupled to the base 200 by a capacitor 202. Resistor 203 is provided to forward bias the base-emitter diode of transistor 198. A diode 204 is connected between the base 200 and ground. A resistor 205 is the load.

The function of the square wave shaper 91 is to convert the sinusoidal output signal from the emitter follower 90 to a square wave with the same zero crossing points as those of the sinusoid. Positive excursions of the sinusoid are shunted to ground through diode 204, thereby cutting off transistor 198. Negative excursions of the sinusoid have no effect on the circuit, inasmuch as transistor 198 is forward biased to saturation and the collector 201 is essentially at ground potential. The voltage across the load resistor 205 is thus a symmetrical square wave, the magnitude of which swings from ground potential to the valueof the collector supply voltage, i.e. -30 volts.

A portion of the output signal from the square wave shaper 91 is fed to the phase detector circuit 86, While the remainder of said signal is fed to a phase inverter 92 and hence to the phase detector circuit 86. The phase inverter 92 comprises a PNP transistor 206 having an emitter 207, a base 208, and a collector 209. Resistor 210 is provided to establish the D.C. operating point, resistor 211 is the load resistor, and resistor 212 is an unbypassed emitter resistor. The signal is coupled to the base 208 of transistor 206 by a capacitor 213. The signal across the load resistor 211 is out of phase with the input signal.

The phase detector circuit 86 broadly comprises PNP transistors 214 and 215 and NPN transistors 216 and 217. Transistors 214 and 215 have emitters 218 and 219, bases 220 and 221, and collectors 222 and 223, respectively. Transistors 216 and 217 have emitters 224 and 225, bases 226 and 227, and collectors 228 and 229, respectively. Connected between the source 102 and a source 230 of positive D.C. potential equal in magnitude to source 102 is a voltage divider network comprising resistors 231, 232, 233, and 234. It is essential for resistor 231 to be equal to resistor 234 and resistor 232 to be equal to resistor 233. In the instant device, each of said resistors has a value of K ohm. The collectors 222 and 223 are connected to the junction 235 of resistors 231 and 232. The collectors 228 and 229 are connected to the junction 236 of resistors 233 and 234. The phase detector output is taken from the junction 237 of resistors 232 and 233. Junction 237 is connected to an integrating network 238 comprising a capacitor 239 and a resistor 240.

The emitters 218, 219, 224, and 225 are all connected directly to ground. The bases 220 and 221 are supplied with sufficient bias current through resistors 241 and 242, respectively, to drive transistors 214 and 215 into saturated conduction in the absence of an input signal. The bases 226 and 227 are supplied with sufiicient bias current through resistors 243 and 244, respectively, to'drive transistors 216 and 217 into saturated conduct-ion in the absence of an input signal. The output signal from the wave shaper 91 is coupled to the base 226 of transistor 216 by a capacitor 245. The output signal from the phase inverter 92 is coupled to the base 220 of transistor 214 by a capacitor 246. Positive half cycles of the output signal from the Schmitt trigger 85 are coupled to the base 221 of transistor 215 by a diode 247 and a current limiting resistor 248. Negative half cycles of the output signal from the Schmitt trigger are coupled to the base 227 of transistor 217 by a diode 249 and a current limiting resistor 250. Diodes 247 and 249 are utilized to isolate transistor 215 from transistor 217, inasmuch as the input signal to both transistors is fed along a common conductor 251. In lieu of the diodes 247 and 249, it is obvious to one skilled in the art that the output signal from the Schmitt trigger 85 can be divided into separate positive and negative waves and individually coupled to the bases 221 and 227 of the transistors 215 and 217, respectively.

FIGURE 10 shows the time relationships of the various signals fed into the phase detector 86 when the probe 52 is properly positioned with respect to the seam 12. FIG. 10A shows the square wave output of the Schmitt trigger 85. The positive half cycle 252 fed to the base 221 of transistor 215 is sufficiently large to reverse bias transistor 215 beyond cutoff. The negative half cycle 253 fed to the base 227 of transistor 217 is sufficiently large'to reverse bias transistor 217 beyond cut-off. FIG- URE 10B shows the positive pulse output signal 254 of the phase inverter 92. The signal 254 is sufficiently large to reverse bias transistor 214 beyond cut-off. FIGURE 10C shows the negative pulse output signal 255 of the wave shaper 91. The signal 255 is sufficiently large to reverse bias transistor 216 beyond cut-off.

The operation of the phase detector circuit 86 is substantially as follows. Transistors 214 and 215 function as parallel switches which, when either of which is closed (i.e. when either transistor is in saturated conduct-ion), essentially ground junction 235 through the saturated transistor. Transistors 216 and 217 also function as parallel switches which, when either of which is closed, essentially ground junction 236 through the saturated transistor.

The input signal from the Schmitt trigger 85 alternately cuts off transistors 215 and 217, while the input signals from the phase inverter 92 and the wave shaper 91 simultaneously cut off transistors 214 and 216. From time t to t transistor 215 is cut off and transistors 214, 216, and 217 are in saturated conduction. Thus both junctions 235 and 236 are grounded during this time interval. Junction 237 which is located at the midpoint of the voltage divider network, is therefore also grounded. From time i to 1 transistors 214, 215, and 216 are cut off and transistor 217 is in saturated conduction. Therefore junction 236 remains at ground potential during this time interval but the potential at junction 235 changes from zero to a value substantially equal to two-thirds of the potential of the negative source 102. The potential at junction 237 changes correspondingly from zero to a value substantially equal to one-third of the potential of the negative source 102. The signal 256 at junction 237, in the absence of the integrating network 233, during this time interval is shown in FIG. 10D.

At time t the polarity of the input signal from the Schmitt trigger substantially instantaneously reverses from positive to negative. This polarity reversal results in the return of transistor 215 to saturated conduction and the cutting off of transistor 217, while the conduction states of transistors 214 and 216 remain the same as during the time interval t to. t From times t to t transistors 214, 216, and 217 are cut off and transistor 215 is in saturated conduction. Thus junction 235 is grounded during this time interval. The potential at junction 236 changes from zero to a value substantially equal to two-thirds of the potential of the positive D.C. source 230. The potential at junction 237 in the absence of the integrating network 238, changes correspondingly from zero to a value substantially equal to one-third of the potential of the positive source 230.

At time t transistors 214 and 216 substantially instantaneously return to a state of saturated conduction and junctions 235, 236, and 237 are all at ground potential during the time interval from t to t.,. Inasmuch as the time interval from t to 2 equals that from 1 to t the voltage across the integrating network 238 during the entire time interval from t to L; is equal to Zero.

FIGURE 11 shows the waveforms of the signals fed into the phase detector 86 at an instant when the probe 52 is not properly positioned with respect to the seam 12 of the pipe 11. The output signal from the Schmitt trigger 85, shown in FIG. 11A, is unchanged. However, the output signals from the phase inverter 92 and the wave shaper 91, shown in FIGS. 11B and 11C, are shifted in time by an amount and in a direction dependent upon the magnitude and direction of the shift of the seam 12 with respect to the probe 52. The output signals from the phase inverter 92 and the wave shaper 91 exist from t to t The resultant potential at junction 237, in the absence of the integrating network 238, is shown in FIG. 11D. The actual voltage output from the phase detector circuit 86 is shown in FIG. 11E.

The DC. voltage across the integrating network 238 fully describes the position of the probe 52 with respect to the seam 12. In the preferred embodiment of the invention, said voltage is fed to a difference amplifier 93. The difference amplifier 93 comprises two PNP transistors 257 and 258 having emitters 259 and 260, bases 261 and 262, and collectors 263 and 264, respectively. The emit" ters 259 and 2611 are connected to a junction 265 through current limiting resistors 266 and 267. The junction 265 is connected to a potentiometer 268 which is in turn connected to the source 230. The collectors 263 and 264 are connected together through a microammeter 269 and a current limiting resistor 270. The microammeter 269, which is of the Zero center type, provides means for visually indicating the relative position of the probe 52 with respect to the seam 12. The collectors 263 and 264 are further connected to a pair of sensitive relays 271and 272 which are adapted to energize the reversible A.C. motor 21. The relays 271 and 272 are connected to a common junction 273 which is connected to the source 162 by a conductor 274. A current limiting resistor 275 is connected from the base 262 of transistor 258 to ground. The signal across the integrating network 238 is fed through a conductor 276 and is couplied to the difference amplifier 93 by means of a potentiometer 277. A filter capacitor 27 8 is provided to remove any A.C. components from the incoming signal.

The difference amplifier 93 operates in substantially the following manner. Potentiometer 268 is adjusted to obtain the desired quiescent collector currents and potentiometer 277 is adjusted to control the level of the input signal required for one of the relays 271 and 272 to close. As the input current to the base 261 of transistor 257 increases negatively, the current through transistor 257 increases and the current through transistor 258 decreases. A sufficiently large current through transistor 257 causes relay 271 to close. Conversely, as the input current to the base 261 of transistor 257 increases positively, the currentthr-ough transistor 257 decreases and the current through transistor 258 increases. A sufficiently large current through transistor 258 causes relay 272 to close.

As a specific example of the phase detector circuit, which I believe to be novel, the following components may be utilized:

Transistors 214 and 215 2Nll25 Transistors 216 and 217 2N35 Diodes 247 and 249 1N34A Resistors 231, 232, 233, and 234 ohms 10K Resistor 240 do 68K Resistors 241, 242., 243, and 244 do 100K Resistors 248 and 250 do 10K Capacitor 239 mfd .05 Capacitor 245 mfd .033 Capacitor 246 mfd .02

It is to be understood that the values for the components of the phase detector circuit may vary in accordance with the particular application of the circuit.

From the above description of the invention and its associated mechanical components and electrical circuits, it is believed the operation of the invention is clear in connection with automatic positioning devices. When the probe 52 is properly positioned with respect to the seam 12 of the pipe 11, alternating current in the excitation coil 74 results in equal fluxes linking the pipe 11 on each side of the seam 12. The eddy currents induced in the pipe 11 are thus equally distributed on each side of the seam 12, and the fluxes resulting from said eddy currents are equal in magnitude and 180 out of phase. The total voltage across the pick-up coils 75 and 76 is thus due only to the electromagnetic inequality of the coils 75 and 76, and the phase angle B is zero.

The input signals to the phase detector circuit 86 from the wave shaper 91 and the phase inverter 92 are therefore 90out of phase with the input signal from the Schmitt trigger 85, and the voltage across the integrating network 238 is zero. The difference amplifier 93 thus receives no input signal, and the motor 21 remains deenergized.

Should the seam 12 of the pipe 11 experience a lateral displacement with respect to the probe 52, alternating current in the excitation coil 74 results in a larger flux linking the pipe 11 on one side of the seam 12 than on the other side of the seam 12. This inequality in flux alters the eddy current distribution on each side of the seam 12, and results in a net voltage induced in the pick-up coils 75 and 76 which is out of phase with theexcitation voltage by an angle B.

The input signals fed to the phase detector circuit 86 from the wave shaper 91 and the phase inverter 92 are therefore out of phase with the input signal from the Schmitt trigger 85 by an angle of 90 +B, and the voltage across the the integra-ting network 238 differs from zero by an amount dependent upon the magnitude of B. The difference amplifier 93 thus receives an input signal and, if the signal is of sufficient magnitude, causes the motor 21 to be energized. Energization of the motor 21 causes the carriage 27, and therefore the probe 52, to be re-positioned until the angle B returns to zero.

While the invention has been described in connection with automatic following devices, there are many other instances in which a DC. signal indicative of a departure from a predetermined eddy current distribution in conductive materials can be advantageously utilized. For example, such a signal can be utilized in the detection of flaws in materials, inasmuch as the electrical and mag- 14 netic properties of the materials in the vicinity of a flaw differ from those where there is no flaw.

Changes and modifications of this invention will und'oubtdly occur to those skilled in the art and I therefore do not wish to be limited to the exact embodiments shown and described but may use such substitutions, modifications or equivalents thereof as are embraced within the scope of my invention or as pointed out in the claims.

I claim:

,1. A circuit for determining any difference in phase between a first and a second alternating signal. having the same frequency, comprising:

(a) a first source of DC. potential, negative in polar- (b) a second source of DC. potential, positive in polarity and equal in magnitude to said first source;

(c) a voltage divider network comprising first, second,

third, and fourth resistors connected in series between said first and second sources;

(d) said first and fourth resistors being equal in magnitude;

(e) said second and third resistors being equal in magnitude;

(f) output means, including integrating means, connected to the junction of said second and third resistors;

(g) first and second switching means connecting the junction of said first and second resistors to ground;

(h) third and fourth switching means connecting the junction of said third and fourth resistors to ground;

(i) means for shifting the phase of said second signal by degrees;

(j) means coupling said phase-shifted second signal to said first and third switching means for simultaneously opening said first and third switching means during alternate half cycles of said phase-shifted second signal;

(k) means coupling one half cycle of said first signal to said second switching means for opening said second switching means during said half cycle; and

(1) means coupling the other half cycle of said first signal to said fourth switching means for opening said fourth switching means during said other half cycle.

2. A circuit as recited in claim 1, in which each of said switching means comprises a transistor biased to saturation in the absence of an input signal.

3. A circuit for determining any difference in phase between a first and a second alternating signal having the same frequency, comprising:

(a) a first source of DC. potential, negative in polar- (b) a second source of DC. potential, positive in polarity and equal in magnitude to said first source;

(c) a voltage divider network comprising first, second, third and fourth resistors connected in series between said first and second sources;

(d) said first and fourth resistors being equal in magnitude;

(c) said second and third resistors being equal in magnitude;

(f) output means, including integrating means, connectedto the junction of said second and third resistors;

(g) first and second switching means connecting the junction of said first and second resistors to ground;

(h) third and fourth switching means connecting the junction of said third and fourth resistors to ground;

(i) means for converting said first alternating signal into a first substantially square wave of the same phase and frequency;

(j) means for converting said second alternating signal into a second substantially square wave of the same frequency and shifted in phase by 90 degrees;

(k) means coupling said second square wave to said first and third switching means for simultaneously opening said first and third switching means during alternate half cycles of said second square wave;

(1) means coupling one half cycle of said first square wave to said second switching means for opening said second switching means during said half cycle; and

(m) means coupling the other half cycle of said first square wave to said fourth switching means for opening said fourth switching means during said other half cycle.

4. A circuit for determining any difference in phase between a first and a second alternating signal having the same frequency, comprising;

(a) a first source of DC. potential, negative in polar- (b) a second source of DC potential, positive in polarity and equal in magnitude to said first source;

() a voltage divider network comprising first, second,

third and fourth resistors connected in series between said first and second sources;

(d) said first and fourth resistors being equal in magnitude;

(c) said second and third resistors being equal in magnitude;

(f) output means, including integrating means, connected to the junction of said second and third resisters;

(g) first and second PNP transistors connected in parallel between the junction of said first and second resistors and ground;

(h) first and second NPN transistors connected in parallel between the junction of said third and fourth resistors and ground;

(i) biasing means connected to said PNP and NPN transistors for driving said transistors into saturated conduction;

(j) means coupling the positive half cycles of said first signal to said first PNP transistor whereby said first PNP transistor is cut off during said half cycles;

(k) means coupling the negative half cycles of said first signal to said first NPN transistor whereby said first NPN transistor is cut off during said half cycles;

(1) means for shifting the phase of said second signal by 90 degrees; and

(m) means coupling said phase-shifted second signal to said second PNP and NPN transistors whereby said second PNP and NPN transistors are simultaneously cut off during alternate half cycles of said signal.

5. A circuit as recited in claim 4, in which means (111) includes means for inverting said phase-shifted second signal, means coupling said inverted signal to one of said second PNP and NPN transistors, and means coupling said phase-shifted second signal to the other of said second PNP and NPN transistors.

6. A circuit as recited in claim 5, in which said integrating means comprises a resistive-capacitive network.

'7. A circuit for determining any difference in phase between a first and a second alternating signal having the same frequency, comprising:

(a) a first source of potential, negative in polarity;

(b) a second source of potential, positive in polarity and equal in magnitude to said first source;

(c) a voltage divider network comprising first, second, third and fourth resistors connected in series between said first and second sources;

((1) said first and fourth resistors being equal in magnitude;

(e) said second and third resistors being equal in magnitude;

(f) output means, including integrating means, connected to the junction of said second and third resistors;

(g) first and second PNP transistors each having an emitter, a base, and a collector;

(h) means connecting said collectors of said first and second PNP transistors to the junction of said first and second resistors;

(i) first and second NPN transistors each having an emitter, a base, and a collector;

(j) means connecting said collectors of said first and second NPN transistors to the junction of said third and fourth resistors;

(k) means connecting said emitters to ground;

(l) biasing means connected to said PNP and NPN transistors for driving said transistors into saturated conduction;

(m) means coupling the positive half cycles of said first signal to the base of said first PNP transistor whereby said first PNP transistor is cut off during said positive half cycles;

(11) means coupling the negative half cycles of said first signal to the base of said first NPN transistor whereby said first NPN transistor is cut off during said negative half cycles;

(0) means for shifting the phase of said second signal by degrees;

(p) means for inverting said phase-shifted second signal;

(q) means coupling said phase-shifted second signal to the base of one of said second PNP and NPN transistors whereby said transistor is cut oif during alternate half cycles; and

(r) means coupling said inverted phase-shifted second signal to the base of the other of said second PNP and NPN transistors whereby said transistor is cut off during alternate half cycles.

8. A circuit for determining any difference in phase between a first and a second alternating signal having the same frequency, comprising:

(a) a first source of potential, negative in polarity;

(b) a second source of potential, positive in polarity and equal in magnitude to said first source;

(c) a voltage divider network comprising first, second, third and fourth resistors connected in series between said first and second sources;

(d) said first and fourth resistors being equal in magnitude;

(c) said second and third resistors being equal in magnitude;

(f) output means, including integrating means, connected to the junction of said second and third resisters;

(g) first and second PNP transistors each having an emitter, a base, and a collector;

(b) means connecting said collectors of said first and second PNP transistors to the junction of said first and second resistors;

(i) first and second NPN transistors each having an emitter, a base, and a collector;

(j) means connecting said collectors of said first and second NPN transistors to the junction of said third and fourth resistors;

(k) means connecting said emitters to ground;

(1) biasing means connected to said PNP and NPN transistors for driving said transistors into saturated conduction;

(m) means for converting said first alternating signal into a first substantially square wave of the same phase and frequency;

(11) means coupling the positive half cycles of said first square wave to the base of said first PNP transistor whereby said first PNP transistor is cut off during said positive half cycles;

(0) means coupling the negative half cycles of said first square wave to the base of said first NPN tran- 17 18 sistor whereby said first NPN transistor is cut ofi' (s) means coupling said inverted second square wave during said negative half cycles; to the other of said second PNP and NPN transis- (p) means for converting said second alternating sigtors whereby said transistor is cut 01f during alternal into a second substantially square wave of the nate half cycles.

same frequency and shifted in phase by 90 degrees; 5 (q) means coupling said second square wave to the base of one of said second PNP and NPN transistors whereby said transistor is cut ofit' during alter- ARTHUR GAUsSPrmary Exammernate half cycles; R. H. EPSTEIN, Assistant Examiner. (r) means for inverting said second square wave; and 10 No references cited. 

1. A CIRCUIT FOR DETERMINING ANY DIFFERENCE IN PHASE BETWEEN A FIRST AND A SECOND ALTERNATING SIGNAL HAVING THE SAME FREQUENCY, COMPRISING: (A) A FIRST SOURCE OF D.C. POTENTIAL, NEGATIVE IN POLARITY; (B) A SECOND SOURCE OF D.C. POTENTIAL, POSITIVE IN POLARITY AND EQUAL IN MAGNITUDE TO SAID FIRST SOURCE; (C) A VOLTAGE DIVIDER NETWORK COMPRISING FIRST, SECOND, THIRD, AND FOURTH RESISTORS CONNECTED IN SERIES BETWEEN SAID FIRST AND SECOND SOURCES; (D) SAID FIRST AND FOURTH RESISTORS BEING EQUAL IN MAGNITUDE; (E) SAID SECOND AND THIRD RESISTORS BEING EQUAL IN MAGNITUDE; (F) OUTPUT MEANS, INCLUDING INTEGRATING MEANS, CONNECTED TO THE JUNCTION OF SAID SECOND AND THIRD RESISTORS; (G) FIRST AND SECOND SWITCHING MEANS CONNECTING THE JUNCTION OF SAID FIRST AND SECOND RESISTORS TO GROUND; (H) THIRD AND FOURTH SWITCHING MEANS CONNECTING THE JUNCTION OF SAID THIRD AND FOURTH RESISTORS TO GROUND; (I) MEANS FOR SHIFTING THE PHASE OF SAID SECOND SIGNAL BY 90 DEGREES; (J) MEANS COUPLING SAID PHASE-SHIFTED SECOND SIGNAL TO SAID FIRST AND THIRD SWITCHING MEANS FOR SIMULTANEOUSLY OPENING SAID FIRST AND THIRD SWITCHING MEANS DURING ALTERNATE HALF CYCLES OF SAID PHASE-SHIFTED SECOND SIGNAL; (K) MEANS COUPLING ONE HALF CYCLE OF SAID FIRST SIGNAL TO SAID SECOND SWITCHING MEANS FOR OPENING SAID SECOND SWITCHING MEANS DURING SAID HALF CYCLE; AND (L) MEANS COUPLING THE OTHER HALF CYCLE OF SAID FIRST SIGNAL TO SAID FOURTH SWITCHING MEANS FOR OPENING SAID FOURTH SWITCHING MEANS DURING SAID OTHER HALF CYCLE. 